Stirling in Deep Space

For more than 30 years now, NASA’s deep-space probes have relied on radioisotope thermoelectric generators (RTGs), devices that use decaying plutonium 238 to warm thermocouples and generate electricity. Now the space agency is poised to replace those heavy, expensive and inefficient RTGs with a system that provides more power with much less radioactive fuel—technology based on a 19th-century invention.

Patented in 1816 by an intellectually restless Scottish minister named Robert Stirling, the Stirling engine is simplicity itself: two chambers or cylinders, one cold and one hot, containing a “working fluid” (commonly air, helium or hydrogen) with a regenerator or heat exchanger between the two. Differences in temperature and pressure between the two cylinders cause the working fluid to expand and contract, passing back and forth through the exchanger and moving a piston. The process thereby converts thermal energy (in NASA’s case, supplied by radioactive decay) into mechanical energy.

“Stirling is actually something we’ve been investing in for almost the past three decades at some level, and it’s now reached the point where we’re ready to make the next step forward,” says Dave Lavery, one of the directors of the Solar System Exploration program at NASA headquarters in Washington, D.C.

Lockheed Martin is putting the finishing touches on an engineering test unit that should be ready this spring: the advanced Stirling radioisotope generator. Two Stirling converters inside the generator drive pistons within a linear alternator, generating about 100 watts of electrical power. The unit will be less than a yard long and a foot wide, small enough to fit in the backseat of a subcompact car and weighing just over 40 pounds—less than half the weight of a typical RTG. It will boast a conversion efficiency of 20 to 30 percent, compared with the measly 6 to 7 percent of RTGs—while requiring only one-fourth the amount of radioisotope fuel.

Those characteristics translate into important advantages for spaceflight. Because the Stirling unit will be less massive and thus cheaper to launch, it will allow a spacecraft to carry a larger payload. The fourfold reduction in radioactive fuel—from 20 pounds in an RTG to five in a Stirling—also saves money while considerably reducing safety concerns involved in a worst-case scenario of a launch vehicle exploding in midair. NASA is keenly aware of public concerns about radiological safety, and as Lavery puts it, “for any nuclear-based system, we go through the entire National Environmental Policy Act process,” which requires NASA to collect public comments before any final launch decision.

Richard Shaltens, chief of the Thermal Energy Conversion Branch at the NASA Glenn Research Center, explains that once Lockheed completes initial testing, NASA Glenn will put the device through extended evaluations to begin its transition to flight status. “We’re planning to go forward with the potential use of this technology on future missions in probably the 2012–2013 time frame,” Shaltens says. He also points out that in more than 100,000 hours of lab testing in various environments, the Stirling converters “have demonstrated that they perform as predicted and have the potential for long life” comparable to RTGs.

NASA is so confident in the Stirling radioisotope generator that the agency has already invited the space science community to submit Stirling-based planetary mission concepts. Lavery emphasizes that the generator’s inaugural mission will not be decided until at least 2009, but possible jobs include flights to the outer planets and manned missions to the moon or Mars. “Their overall design right now is to be compatible with either a deep-space interplanetary environment or planetary-surface environment with either atmosphere or vacuum,” Lavery says.

Eventually Stirling technology may phase out RTGs completely. Lavery expects that it “would be the beginning of a new family of [radioisotope power systems] that are significantly more efficient and significantly less costly than the solutions we had available so far.” Reverend Stirling could hardly have imagined that his ingenious invention might well become the prime mover that powers the next great era of solar system exploration.